1. Field of the Invention
This invention relates to a method for producing glass-less grain-oriented electrical steel sheet exhibiting metallic luster and more particularly to such a steel sheet having very superior punching property.
2. Description of the prior art
Grain-oriented electrical steel sheet is generally produced as follows. A silicon steel slab containing not more than 4.0% Si is hot rolled, optionally annealed, subjected to a single cold rolling or two or more cold rollings alternately with intermediate annealing, and then cold rolled to the final sheet thickness. It is then subjected to decarbonization annealing to produce a SiO.sub.2 -containing oxide film on its surface, whereafter an annealing separator consisting mainly of MgO is applied to the surface thereof and dried. The sheet is then wound into a coil and subjected to high-temperature finish annealing. As a result, the formation of Goss texture secondary recrystallization grains is promoted and a glass film is formed on the sheet surface. If required, the sheet is coated with an insulating coating liquid and then baked to form an insulating film thereon.
Grain-oriented electrical steel sheet is used as a material for the iron cores of dynamos, transformers and other electrical equipment. Ordinarily such a core is fabricated by punching or shearing the sheet into appropriately shaped core plates by use of a steel die and then laminating the core plates to obtain the core. In fabricating an iron core for a turbine generator, for example, the number of core plates required amounts to 100 to 200 thousand. Moreover, it is necessary to hold the punching burr height of these plates to not more than, for instance, 15 .mu.m. This is important for the purpose of preventing an abnormal increase in core loss caused by shorting of the edges of the core plates after they have been laminated to form the core.
The surface of grain-oriented electrical steel sheet is coated with a glass film or with both a glass film and an insulating film. As the glass film has high hardness, it increases the wear of the die during the punching operation. For this reason, punching burr tends to occur after several thousand punching operations and it becomes necessary to repolish the die or to replace it with a new one. This greatly reduces the efficiency of the work and leads to an increase in cost.
A method for producing grain-oriented electrical steel sheet having metallic luster is disclosed, for example, in Japanese Published Unexamined Patent Application No. 53(1978)-22113. According to the disclosed method, the thickness of the oxide film produced during decarbonization annealing is held to not more than 3 .mu.m and fine alumina powder blended with 5-40% of hydrated silica mineral powder is used as the annealing separator. After being coated with this separator, the steel sheet is finish annealed. This method produces certain good effects such as that a thin oxide film is obtained, the presence of the hydrated silica makes it possible to form a glass film that separates easily, and the product has a metallic luster.
As one known annealing separator which suppresses the formation of a glass film, there is known that disclosed in Japanese Published Unexamined Patent Application No. 55(1980)-89423 which uses an annealing separator consisting of fine alumina powder blended with 5-30% hydrated silica mineral powder, an Sr compound, a Ba compound, calcium oxide and calcium hydroxide, the annealing separator being applied to the steel sheet prior to finish annealing. As another there is known that disclosed in Japanese Published Unexamined Patent Application No. 561981)-65983 which uses an annealing separator consisting alumina hydroxide blended with of 20 parts by weight of an impurity removing additive and 10 parts by weight of a suppressing agent, the separator being applied to the steel sheet to form a thin glass film of a thickness of 0.5 mm or less.
Further, Japanese Published Unexamined Patent Application No. 59(1984)-96278 discloses an annealing separator consisting of Al.sub.2 O.sub.3, which has low reactivity toward the SiO.sub.2 in the oxide film formed during decarbonization annealing, and MgO calcined at more than 1,300.degree. C. to reduce its reactivity. The method of this application produces a useful effect in that it suppresses the formation of a forsterite film (glass film).
On an actual production line, however, fluctuations sometimes arise in the dew point or constitution of the gas atmosphere during decarbonization annealing, for example. Also, local variations arise in the thickness of the oxide film formed on the surface of the steel plate. Furthermore, depending on its history, the steel plate itself undergoes variation in oxidation in the widthwise or lengthwise direction. On the other hand, methods for removal of glass film are known but entail the risk of non-uniform removal. At any rate, up to the present there has been no method for consistent production of grain-oriented electrical steel sheet exhibiting metallic luster.